Modulation system



May 28, 1940. R L, DWE 2,202,576

MODULATION SYSTEM Filed Jan. 31, 1938 A INVENTOR.

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UNITED sm-es 2,202,576 monntainou srs'ranr Robert L. Drake, Chicago, Ill., assignor to Bendix Radio tion of Delaware orporation, Chicago, 111.; a corporam ma January 31, 1938, Serial No. 187,791

4 Claims. '(cl. ire-171.5

This invention relates ingeneral tocircuits for electric discharge devices and more particularly to circuits in which the character of the output currents is utilized to determine or limit the flow of space current within the'device. The invention also relates particularly to a pushpull type of circuit in which a pair of discharge devices are maintained in balanced relationship.

An object of the invention is to provide a cir- 0 cuit wherein the output oi. anelectric discharge device is determined by the presence of some predetermined frequency at the input of the device.

Where,. for example, the input to a vacuum tube, includes alternating voltages of diiierent fre- 15 quencies, it may be desired to cut down the total output of the-circuit when a voltage of some one frequency is present; and such may be accomvplished by a circuit of this invention.

Another important object of the invention is 26 to provide means for automatically balancing apair oi'discha'rge devices when these are con-,

nected in push-pull relation. When ubes are connected in push-pull arrangement, it is necessary for good operation that theme currents 25 passed by the tubes be equal in magnitude, and

if this is not the case distortion or faulty operation will result. It is desiredto prevent unbalanced operation even though the tubes shoul have different gain characteristics.' g 30 A further object is to provide means for correcting the unbalance of discharge devices in push-pull arrangement throughcontrol eilected 1 by currents of one frequency; this means being unaffected by currents of other than thi's fre-f 1 quency. It also desired that the control for 35 eifecti'ng equalization between the devices be sensitive so as to efiect an exact balance and prevent all error due to-unbalances' as far asthis is possible. Other objects and advantages will beapparent later in the specification. Embodiments of the invention are illustrated in the,

- and B.

Figure 2 is a circuit diagram of a balanced: modulator constituting another embodiment of are connected together and to ground-through" the control circuit C.- Circuit C comprises an inductor l0 and a capacitor ll connected in --parallel relation between ground and cathodes.

The-anodes oftubes A and B are connected to gether through the primary of the output transformer 12, and the mid point of this primarywinding is connected through the plate battery ii to ground. The output circuit of tube A may be followed from the potential source I3, through half of the primary winding. on transformer l2 to the anode of tube A, and from the cathode of A through circuit C to the other terminal of the potential source. Similarly the output circuit of tube B may be followed from the potential source l3, through half of the primary winding on transformer I! to the anode-of tube B, and from the cathode-of B through circuit C to the other terminal of the potential source.

I The input to tubes A and]; is derived from two circuits, one of which may contain voltages of carrier frequency and the other of which may contain voltages of themodulating frequency. The carrier frequency circuit, designated as S1 in Figure 1, .has its one side grounded and has its other side connected through blocking coridensers I4 and L5 .to the grids of both tubes A The other circuit for providing the modulating frequency and designated in the drawingas S2, has therein .an-altemator i6 and 6- includes the primary of transformer H. The alternator I8 is illustrated merely to'show a source of alternating current and any means may be used which will provide the desired modulating frequency. The secondary of transformer I'I has its end terminals connected through resistors l8 to the grids of tubes A and 2B, the resistors being provided to substantially prevent the flow of carrier current in the path the grid circuits of tubes A and B in other suit- Considering first the. carrier-frequency input of the tubes, it willbe seen that the carrier input circuit may be followed from the side of cir cuit s1 not grounded to thegrids of both tubes A and B, then through circuit C, andreturn by ground to the other side of the carrier circuit Sr. As here shown the carrier impulses are fed to each of tubes A and.B in the same phase;

that1is, a positive impulse of this carrier will appear simultaneously on the grids of each tube. The input circuits of modulating frequency may be traced from the secondary N3 of transformer I! to the grid of tube A, and from the cathode of this tube through circuit C, and return to I winding lls through battery l9; and similarly from the other terminal of winding Us to the grid of tube B, and. from the cathode of this tube through winding C, and return to winding I'Is through battery l9.

It will be apparent that this modulating frequency is impressed on tubes A and B in opposite phase relation and tubes A and B will be rendered more conducting at alternate half cycleof the modulating frequency.

The control circuit C is tuned to be resonant at the modulating frequency and its impedance therefore increases veryrsharply when there exists in the common output circuit path of the two.

tubes a current of the modulating frequency. It

is an important feature of this invention that the a voltage drop across this parallel resonant circuit.

is impressed in the grid-cathode circuit of the tubes in such a manner as to equalize the space currents in the two tubes. This function of circuit C will appear more clearly after consideration of the current and voltage conditions in the apparatus.at initially unbalanced condition.

Assume, for the purpose of explanation, that tubes A and B have different gains, the tube A having a tendency toward greater amplification of impulses. Because the modulating frequency is fed to tubes A and B in opposite phase relationship, the modulating frequency component of the tube outputs will be in opposite phase relationship, and the current of this frequency flowing between the cathode of one tube and ground will oppose the current of this frequency flowing .rietween the cathode of the other tube and this frequency flowing through circuit C to be equivalent to the difference of the space currents of this frequency in the two tubes. The difference ground, causing the magnitude of the current of in space currents of the modulating frequency, then, is made to build up a voltage across circuit C which is substantial. This alternating voltage is 180 degrees out of phase with respect to the modulating frequency on the grid of tube A, and I in phase with the modulating frequency on' the grid of tube B. Consequently, this voltage is subtractive with respect to the modulation potential on the gridof tube A, and additive with respect to the modulation potential on the grid of tube B. Since the negative swings of the modulation potential may carry the grids to the plate current cutoff region, the negative half cycles may be Y disregarded. The positive swings of modulation potential on the grid of'tube A will be reduced in amplitude by the value of the voltage developed across circuit C, and positive swings on tube B will be increased by this value of developed voltage. -This adjustment causes the space current in tube A to diminish and the space current in 'tube B to increase and serves to bring the space currents flowing in each of the tubes to equal magnitude which is the condition of balance.

The circuit C is very sensitive to even a slight unbalance, for only a small difference in space currents in the two tubes is effective to build up a substantial voltage across it, due to the fact that the parallel resonant circuit has a very high impedance at the resonant frequency.

From the above explanation it will be apparent. that when the tubes are in themselves-perfectly balanced, the circuit C will have no effect, nor

will it present any substantial impedance which diminishing the space current "impedance across circuit C, and thegrid-cathode would be detrimental to the operation of the apparatus; but that when the tubes are not in themselves perfectly balanced, the circuit C will operate automatically to insure balance through 5 control by the modulating frequency and without impeding the flow of carrier frequency current in the output circuit other than for balancing it between the two tubes.

The modification shown in Figure fi demonl0- strates a second scheme for balancing the.out-

puts of tubes connected in push-pull relation. In'

this arrangement, the cathodes are connected together and the anodes are connected through the primary of the output transformer in the same 15 manner as described in connection with the embodiment of Figure 1. In the path connecting the cathodes of the tubes with the mid point of the output transformer winding is the potential source 30 which supplies plate potential, and also 20 the primary. of transformer 3|. ,The secondary of transformer has its terminals connected to the inner ends of secondary'windings 32 of the transformer 33 corresponding with transformer ll of the first described embodiment. The mid point 25 of the secondary of transformer3l is connected through the biasing battery 34 to ground. In this arrangement a vacuum tube might be substituted for the transformer 3| as the translating means coupling this branch of the output to 30 the grid circuits. Preferably the translating means used here should be such as to cause only small differences of space current in the two tubes to produce substantial differences in the grid volt-' ages, and this is effected by having a translating .device which has the higher terminal voltage at,

its secondary side.

Clearly there will be current flowing through the primary side of the translating means only when the space currents of tubes 'A and B are 4" not equal in magnitude, and voltages will be built up at the secondary of the translating means to equalize the space currents only to such an ex- 1 tent as is necessary to effect balanced conditioh.

Figure 3 illustrates a further modification of the inventionemploying only one electric discharge device. The input to the apparatusashere shown is. through the primary of the input transformer 40. The grid or input circuit of the tube T includes biasing battery 4|, the secondary M of input transformer 40, the circuit C, and the, cathode of tube T. The output circuit of the tube includes anode 'and cathode of the tube, plate battery 42, the primary of, output trans former 43 and circuit C.

This type of circuit may be used wherever it is desired to cut 011 or diminish the output of the tube when some certain frequency is fed into it. Whenever the fre-" quency to which the circuit C is tuned to be resonant, is fed into the input of the tube, this sets up a relatively high voltage dropacross this circuit Which-in turn makes the grid of the tube more negative with respect to the cathode, thus which can pass between anode and cathode." When any other frequency is fed into the tube T, there is very low invention may be constructed and many changes may be made in the construction of the embodiments herein described without departing from the spirit of the invention.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, but the appended claims should be construed as broadly as permissible in view of the priorart.

I claim:

1. In modulating apparatus, -a pair of electric discharge devices arranged in push-pull relation,

each of said devices having an anode, a cathode and a grid, means for varying at one frequency I the potentials of said grids with respect to their associated cathodes and in the same phase .relation, means for .varying the potentials of said grids with respect to their associated cathodes in different phase relation and at another frequency,

and means responsive only to space current insaiddevices of substantially said other frequency for equalizing the total space current flowing in said devices.

2. In modulating apparatus, a pair of electric discharge devices arranged in push-pulhrelation, each of said devices having an anode, a cathode,

and a grid, means for varying at one frequencythe potentials of said grids with respect to their associated cathodes, means for varyinfl' the potentials ofsaid grids with respect to their asso- 3. Apparatus as set forth in claim 2 in which said electrical circuit includes an inductance and a capacitance arranged in parallel in the anodecathode circuits of said devices, said circuit being parallel resonant at said other frequency.

4. In modulating apparatus, a' pair of electric discharge devices each having an anode, a cathode and a grid, a conductive path connecting said cathodes, a conductive path connecting said anodes, a third path connecting each of said .paths, said third path includinga source of potential and a control circuit, said control circuit being parallel resonant at a predetermined frequency and input circuits for each of said devices, each of said input circuits including said control circuit and the grid and cathode of one of said devices, said control circuit being responsive to a diflerence inspace current in said devices of said predetermined frequency to alter the potential of said grids with respect to their.

associated cathodes for equalizing space current in said devices.

ROBERT L. DRAKE. 

